Nippon Steel Corporation

Teruki Hayashida

Tatsuo Yokoi

Akifumi Sakakibara

Jun Ando

Hiroshi Shuto

Osamu Akisue

Masahiro Ohara

Teruaki Yamada

Kazuya Ootsuka

Mutsumi Sakakibara

Toshiyasu Ukena

Daisuke Ito

Daisuke Maeda

Natsuko Sugiura

Takaaki Nakamura

Shinsuke Kai

Takehiro Nakamoto

Masahiko Oda

Takehiro Hoshino

Kenji Kawai

Yuuji Sano

Kouichi Tsuchihashi

A hot-rolled steel sheet has, as a chemical composition, by mass %: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, in which among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an EPMA in a region of 250 μm×250 μm on a surface, the percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.

Hot-rolled steel sheet and method for manufacturing same

In a hot-rolled steel sheet, a microstructure contains, in area ratio, at a location at ¼ a sheet thickness, a total amount of 90% or more of one or more of martensite and tempered martensite and a total amount of 10% or less of one or more of ferrite, bainite, and pearlite, at a location of 200 μm from the surface, a total amount of 70% or more and less than 90% of one or more of martensite and tempered martensite and a total amount of more than 10% and 30% or less of one or more of ferrite, bainite, and pearlite.

Hot-rolled steel sheet

This hot-rolled steel sheet has a predetermined chemical composition, and in a case where the thickness is denoted by t, the metallographic structure at a t/4 position from the surface contains one or both of tempered martensite and lower bainite at a volume percentage of 90% or more, the tensile strength is 980 MPa or more, and the average Ni concentration on the surface is 7.0% or more.

This hot-rolled steel sheet has a predetermined chemical composition, in which in a case where the thickness is denoted by t, a metallographic structure at a t/4 position from the surface includes, by area fraction, 77.0% to 97.0% of bainite or tempered martensite, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, in the metallographic structure, the average grain size excluding the residual austenite is 7.0 &#x3bc;m or less, the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0&#xd7;10carbides/mmor more, a tensile strength is 980 MPa or more, and an average Ni concentration on the surface is 7.0% or more.

The present invention provides a hot-rolled steel sheet capable of preventing softening of the strength of a sheet-thickness central portion of the steel sheet in thermal treatment, even in the case where an amount of working performed on the steel sheet is small and a work hardening rate is low. A hot-rolled steel sheet of the present invention consists of chemical components of, in mass %, C: 0.040 to 0.150%, Si: 0 to 0.500%, Mn: 0.10 to 1.50%, P: 0 to 0.050%, S: 0 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010 to 0.0060%, Nb: 0.008 to 0.035%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Cr: 0 to 0.02%, Mo: 0 to 0.020%, V: 0 to 0.020%, Ca: 0 to 0.0100%, B: 0 to 0.0050%, and the balance: Fe and impurities. The hot-rolled steel sheet contains 0.005 to 0.030% dissolved Nb. An area fraction of ferrite structure is 85% or more, the balance is cementite and/or pearlite structure, and an average crystal grain size of ferrite is equal to or more than 5 &#x3bc;m and equal to or less than 20 &#x3bc;m.

Hot-rolled steel sheet, steel material, and method for producing hot-rolled steel sheet

A hot-rolled steel sheet including predetermined components, in which the amounts of Cr and Al added satisfy Expression (1) below, a metallographic structure has, by % by volume, a ferrite fraction of more than 90% and 98% or less, a martensite fraction of 2% to less than 10%, and, a fraction of a residual structure made of one or more of pearlite, bainite, and residual austenite being less than 1%, the ferrite has an average circle-equivalent diameter of 4 &#x3bc;m or more and a maximum circle-equivalent diameter of 30 &#x3bc;m or less, and the martensite has an average circle-equivalent diameter of 10 &#x3bc;m or less and a maximum circle-equivalent diameter of 20 &#x3bc;m or less.[Cr]&#xd7;5+[Al]&#x2265;0.50&#x2003;&#x2003;Expression (1) Here, in Expression (1), [Cr] represents an amount of Cr (mass %), and [Al] represents an amount of Al (mass %).

The present invention provides a high strength hot rolled steel sheet raised in Si content, wherein a conversion coating can be formed homogeneously over the entire surface of the steel sheet, no new step is added in the production of the steel sheet, and quality control is also facilitated, comprising, by mass %, C: 0.03 to 0.15%, Si: 0.8 to 3.0%, Mn: 0.5 to 3.0%, P: 0.07% or less, S: 0.01% or less, Al: 0.015 to 0.1%, N: 0.001 to 0.008%, and, if necessary, having Ti, Nb, etc. added, the oxides on the steel sheet surface having an Si concentration of 3.5% or less and an Mn concentration of 3.5% or less. Preferably, the average roughness Ra is 3.0 &#x3bc;m or less and the pitting due to pickling is an average of 5 or less in 10 &#x3bc;n side squares. The scale after the hot rolling is washed off by dipping the sheet in a solution having an HCl concentration of 7 to 15% and an Fe ion concentration of 4 to 12% at a solution temperature of 80 to 98&#xb0; C. for 40 sec or more.

Hot rolled steel sheet excellent in chemical convertibility and method of production of the same

The present invention provides high burring, high strength steel sheet excellent in softening resistance of the weld heat affected zone and a method of production of the same, that is, high burring, high strength steel sheet excellent in softening resistance of the weld heat affected zone containing, by wt %, C: 0.01 to 0.1%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P&#x2266;0.1%, S&#x2266;0.03%, Al: 0.005 to 1%, N: 0.0005 to 0.005%, and Ti: 0.05 to 0.5% and further containing C, S, N, Ti, Cr, and Mo in ranges satisfying 0%&#x3c;C&#x2212;(12/48Ti&#x2212;12/14N&#x2212;12/32S)&#x2266;0.05%, Mo+Cr&#x2267;0.2%, Cr&#x2266;0.5%, and Mo&#x2266;0.5%, the balance being Fe and unavoidable impurities, wherein the microstructure comprises ferrite or ferrite and bainite.

High burring, high strength steel sheet excellent in softening resistance of weld heat affected zone and method of production of same

The present invention provides a high-strength thin steel sheet drawable and excellent in a shape fixation property and a method of producing the same. For the steel sheet, on a plane at the center of the thickness of a steel sheet, the average ratio of the X-ray strength in the orientation component group of {100}&#x3c;011&#x3e; to {223}&#x3c;110&#x3e; to random X-ray diffraction strength is 2 or more and the average ratio of the X-ray strength in three orientation components of {554}&#x3c;225&#x3e;, {111}&#x3c;112&#x3e; and {111}&#x3c;110&#x3e; to random X-ray diffraction strength is 4 or less. The arithmetic average of the roughness Ra of at least one of the surfaces is 1 to 3.5 &#x3bc;m; the surfaces of the steel sheet are covered with a composition having a lubricating effect; and the friction coefficient of the steel sheet surfaces at 0 to 200&#xb0; C. is 0.05 to 0.2. Further, the present invention also relates to a method of producing said steel sheet, characterized by: rolling a steel sheet having the chemical components specified in the present invention at a total reduction ratio of 25% or more in the temperature range of the Artransformation temperature +100&#xb0; C. or lower.

High-strength thin steel sheet drawable and excellent in shape fixation property and method of producing the same

A zinc-plated steel sheet with an ageing resistance is produced from an Al-killed steel by hot dip type, continuous zinc plating including recrystallization and annealing, where after recrystallization and grain growth, the steel sheet is quenched at a cooling rate of 30.degree..about.250.degree. C/sec from 720.degree..about.600.degree. C. to 310.degree..about.200.degree. C.; after keeping the steel sheet at the same temperature for 0 to 15 seconds, the steel sheet is reheated to a molten zinc bath temperature; then the steel sheet is dipped into the molten zinc bath for zinc-plating; and then the steel sheet is cooled at a cooling rate of 250 to 5.degree. C./sec from that temperature to 350.degree. C.; and then the steel sheet is cooled at a specific average cooling rate in a temperature region of 350.degree. C. to 300.degree. C. and at a specific average cooling rate in a temperature region of from 300.degree. C. to 285.degree..about.220.degree. C.

Process for producing a zinc-plated steel sheet with an ageing

A cold rolled steel sheet having a good ageing resistance is produced by subjecting a cold rolled steel sheet to continuous annealing including recrystallization, grain growth, quenching, supercooling, reheating and overageing according to inclinatory cooling, where after the recrystallization and the grain growth, the steel sheet is quenched at a cooling rate of 50 to 250.degree. C./sec from 720-600.degree. C. to 200-310.degree. C.; after retaining the steel sheet at the same temperature for 0 to 15 seconds, the steel sheet is reheated by at least 40.degree. C. up to 320-400.degree. C.; then the steel sheet is retained at the same temperature or cooled at a rate of not more than 0.7.degree. C./sec including the time for retaining the steel sheet at the same temperature; and then the steel sheet is cooled at an average cooling rate of not more than 10.degree. C./sec in a temperature zone of higher than 350.degree. C., at a specific average cooling rate in a temperature zone of 350.degree. C. to 300.degree. C. and at a specific average cooling rate down to 285-220.degree. C. in a temperature zone of lower than 300.degree. C.

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Oita, Japan

Teruki Hayashida